Lithographic apparatus, device manufacturing method, and method of applying a pattern to a substrate

ABSTRACT

A lithographic apparatus includes at least one image alignment sensor for receiving radiation projected from an alignment mark on a reticle. Processor processes signals from the sensor(s) to resolve spatial information in the projected alignment mark to establish a reference for measuring positional relationships between a substrate support and the patterning location. Examples of the sensor include line arrays of photodetectors. A single array can resolve spatial information in a plane of the sensor (X, Y direction) and in a perpendicular (Z) direction. At least a final step in establishing the reference position is performed while holding the substrate support stationary. Errors and delays induced by mechanical scanning of prior art sensors are avoided. Alternatively (not illustrated) the sensor is moved for mechanical scanning relative to the substrate support, independently of the main positioning systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/908,564, filed Oct. 20, 2010, now allowed, which claims priority andbenefit under 35 U.S.C. §119(e) to U.S. Provisional Patent ApplicationNo. 61/253,630, entitled “Lithographic Apparatus, Device ManufacturingMethod, and Method Of Applying A Pattern To A Substrate”, filed on Oct.21, 2009. The content of each of the foregoing applications areincorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device. The invention further relates to a method ofapplying a pattern from a patterning device onto a substrate, and to acomputer program product for controlling a lithographic apparatus toimplement steps of such methods. The invention is particularly concernedwith the process and apparatus for aligning a substrate table accuratelywith respect to a projected image of alignment marks on the patterningdevice.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusesinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

Whichever type of apparatus is employed, the accurate placement ofpatterns on the substrate is a chief challenge for reducing the size ofcircuit components and other products that may be produced bylithography. In particular, the challenge of measuring accurately thefeatures on a substrate which have already been laid down is a criticalstep in being able to position successive layers of features insuperposition accurately enough to produce working devices with a highyield. So-called overlay must be achieved within a few tens ofnanometers in today's sub-micron semiconductor devices, down to a fewnanometers in the most critical layers.

Consequently, modern lithography apparatuses involve extensivemeasurement operations prior to the step of actually exposing orotherwise patterning the substrate at a target location. Theseoperations, being time-consuming, limit the throughput of thelithography apparatus, and consequently increase the unit cost of thesemiconductor or other products. Various steps have been taken tomitigate these delays in the prior art. For example, an arrangementprovides dual wafer tables, so that two wafers can be loaded in themachine simultaneously. While a first wafer is undergoing exposure in anexposure station, a second wafer is undergoing measurement processes toestablish an accurate ‘wafer grid’ and height map. The apparatus isdesigned so that the tables can be swapped without invalidating themeasurement results, thereby reducing the overall cycle time per wafer.Other techniques to process measurement and exposure steps in parallelmay be employed as well.

One measurement task, which is typically used as a datum forinterpreting many other measurements, is the image alignmentmeasurement, by which a pattern projected by the patterning deviceitself is picked up by sensors coupled, directly or indirectly, to thesubstrate support. These sensors, in combination with sensors measuringrelative positional movements of the substrate table in threedimensions, provide the datum levels by which other measurements can beused to place a desired portion of the substrate accurately in X and Ydirections, and also in a Z (focus) direction. The accuracy andrepeatability of these datum levels, whether individually or instatistical combinations of multiple measurements, is a limiting factorof the overall accuracy of patterning location and focus.

Known image alignment sensors comprise sensors fixed in relation to thesubstrate table (fixed at least for a duration of the image alignmentmeasurement). The positioning subsystem for the substrate table is usedto move the table so as to scan the sensor in X, Y and Z directionsthrough the projected radiation field. By interpreting the measuredintensity of the sensor signals at various values of X, Y and Z, theactual position of the projected image can be derived in terms of theappropriate coordinate system of the positioning subsystem.

In seeking to increase the accuracy of such a system, various obstaclesare encountered. Firstly, the scanning motion for image alignmentinevitably induces vibrations, and therefore inaccuracies in the imagealignment result. To reduce the vibrations by reducing the speed ofscanning would delay the measurement, and could impact overallproductivity (throughput). Moreover, the positioning subsystem for thewafer table is not optimized for the image alignment scanning operation,but rather for the exposure operation. The resulting positioninginaccuracies during the image alignment scan can result in unevensampling of the projected image.

While these errors have been within tolerances for present generations,any source of error will become significant as manufacturers strivetoward the goal of reaching ever-higher levels of positional accuracy.Speed of measurement is also key to improving throughput, while cost ofthe apparatus is also a factor.

SUMMARY

It is desirable therefore to mitigate further the measurement overheadand/or measurement and positioning errors in lithographic apparatus. Aparticular desire is to improve the performance and/or speed of theprojected image measurement.

According to an aspect of the invention, there is provided alithographic apparatus arranged to project a pattern from a patterningdevice onto a substrate, the apparatus comprising:

a patterning subsystem for receiving said patterning device andprojecting said pattern to a substrate held at a patterning location;

a substrate support for holding the substrate while said pattern isapplied;

at least one positioning subsystem for moving said substrate support,said patterning subsystem and said patterning device relative to eachother such that said pattern is applied at an accurately known locationon the substrate; and

a measuring subsystem for measuring the location of said substraterelative to the patterning location, and for supplying measurementresults to said positioning subsystem,

wherein said measuring subsystem includes at least one sensor forreceiving radiation projected from an alignment mark, the sensor andalignment mark being associated one with the patterning device and theother with the substrate support, the processor, and a processor forreceiving and processing signals from the sensor(s) to resolve spatialinformation in the projected alignment mark to establish a reference formeasuring positional relationships between said substrate support andsaid patterning location, and wherein the sensor and the processor areoperable to perform at least a final step in establishing the referenceposition while holding the substrate support and patterning devicestationary with respect to one another.

One option for implementing the apparatus according to this aspect ofthe invention is to provide separate actuators for moving the sensorrelative to the substrate support or the patterning device.

According to other embodiments, said sensor comprises an array ofphotodetector elements separated in at least one dimension, and a signalprocessor for calculating said reference position accurately in at leastone dimension by combining signals representing radiation intensitiesmeasured by the individual elements of the array when the projectedalignment mark falls on the array.

According to some embodiments the processor is arranged to distinguishbetween different elements in accordance with respectively differentoptical path lengths from the alignment mark, thereby to calculate areference position in a dimension (Z) parallel to an optical axis of theprojection system. In the case of optical lithography, the alignmentmark may be projected to the sensor (S) using the same projection systemand the same illumination as projects the product pattern present on thepatterning device. This approach, although strictly optional, isconvenient and brings accuracy and simplicity to the measurementcalculations, but other implementations are feasible.

In the case of imprint lithography, the product pattern is applied moredirectly and is not projected optically. Nevertheless an opticalprojection of the alignment mark may still be deployed between thepatterning device, or its supporting structure, and the substrate or itssupporting structure. In principle, embodiments of the invention appliedto imprint lithography may involve a sensor on the patterning device andmarks projected from the substrate support or an associated element, tothe sensor.

While direct projection between alignment mark and the sensor isillustrated and described in the embodiments that follow, modificationsare envisaged in which for some reason the projection of the alignmentmark is reflected at one or other of the substrate support and thepatterning device or its support, and the sensor and alignment mark areboth at the same side of the projecting optical system.

According to another aspect of the invention, there is provided a devicemanufacturing method comprising projecting a pattern from a patterningdevice onto a substrate, the method comprising:

providing a patterning subsystem for receiving said patterning deviceand applying said pattern to a portion of said substrate held at apatterning location;

holding the substrate on a substrate support;

measuring the location of said substrate relative to the patterninglocation;

operating said patterning subsystem while using results of saidmeasuring step to position said substrate support, said patterningsubsystem and said patterning device relative to each other in asequence of movements such that said pattern is applied at a pluralityof desired portions of the substrate; and

processing said substrate to create, product features in accordance withthe applied pattern,

wherein said measuring step includes a preliminary step of (i) receivingradiation projected from an alignment mark using a sensor and (ii)processing signals from the sensor to resolve spatial information in theprojected alignment mark to establish a reference for measuringpositional relationships between said substrate support and saidpatterning location in at least one dimension, and wherein the sensorand the measuring subsystem are arranged to perform at least a finalstep in establishing the said reference position while the substratesupport and patterning device are held stationary with respect to oneanother.

According to an aspect of the invention, there is provided a computerprogram product containing one or more sequences of machine-readableinstructions for controlling a lithographic apparatus, the instructionsbeing adapted for controlling the measurement and positioning steps of amethod as set forth in any of the aspects of the invention above.

These and other features and advantages of the invention will beunderstood by the skilled reader from a consideration of the exemplaryembodiments discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 is a schematic plan view of a substrate table and substrate inthe apparatus of FIG. 1, showing alignment marks and sensors useful inone embodiment of the invention;

FIGS. 3A to 3C show steps in a known alignment process using thesubstrate table of FIG. 2;

FIG. 4 illustrates the principle of operation of an image alignmentsensor in the step illustrated in FIG. 3C;

FIG. 5 illustrates the arrangement and structure of novel imagealignment marks on a patterning device for use in accordance with oneembodiment of the present invention;

FIG. 6 illustrates the modified apparatus with a novel image alignmentsensor according to various embodiments of the present invention;

FIG. 7 illustrates the principle of operation of an image alignmentsensor according to a first embodiment of the invention;

FIGS. 8, 9 and 10 illustrate various forms and principles of operationof image alignment sensors according to a second embodiment of theinvention;

FIGS. 11 and 12 illustrate modification of the embodiments of FIGS.8-10, to incorporate Z-direction measurement;

FIG. 13 illustrates a second example of a sensor incorporatingZ-direction measurement;

FIG. 14 illustrates two variations on the structure of the sensor foruse in embodiments of the invention;

FIG. 15 illustrates an alternative embodiment in which Z-directionmeasurement is implemented by a modified patterning device rather than amodified sensor;

FIG. 16 illustrates the structure and principle of image alignment markand image alignment sensor in accordance with a third embodiment of theinvention;

FIG. 17 illustrates the provision of sensors for different axes in onesensor block;

FIG. 18 illustrates a combined multi-axis sensor; and

FIG. 19 illustrates computer system hardware useful in implementing themeasurement and exposure processes of FIGS. 4 to 16.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. The inventiondisclosed herein is applicable in both single- and multi-stageapparatuses.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically depicts an arrangement of the substrate table WTdepicted in the lithographic apparatus of FIG. 1, in which the substratetable WT is provided with an embodiment of the image sensor according tothe invention. In the substrate table WT, two image sensors IAS1 andIAS2 are provided. The image sensors can be used to determine a locationof an aerial image of a pattern, e.g. an object mark, on the mask MA byscanning the image sensor IAS1 or IAS2 through the “aerial image” of themarks, as described below. Alignment marks P1 to P4 are distributed alsoon the substrate itself.

The term “aerial image” is used in this context to refer to thepatterned radiation field which would form a real image, were asubstrate or other target present. Features of this field have positionsand extents in the X and Y direction according to the pattern andresolving powers of the projection system, and have a height andvertical extent according to the focus position and depth of focus ofthe projection system. The purpose of the entire process is to positionthe aerial image of the device pattern accurately in X, Y and Zdirections, during the exposure of device patterns in a resist layer orother target.

FIGS. 3A to 3C illustrate steps in the alignment process using the imagealignment sensors IAS1, IAS2. In addition to the parts generally labeledthe same as in FIG. 1 and FIG. 2, an alignment sensor 300 is provided todirect an alignment radiation beam 302 in the direction of the substrateW and/or substrate table WT. At the stage of operation shown in FIG. 3Asensor 300 detects the properties of the beam 302 when reflected, inorder to detect alignment of the sensor 300 to patterns such as P1-P4 onthe substrate. As shown in FIG. 3B, movement of the substrate table WT,which is measured accurately by sensor IF (shown in FIG. 1) can bringalignment of the radiation beam 302 to bear also on alignment mark 304on the substrate table. Mark 304 is accurately placed relative to imagesensor IAS1, IAS2 etc., for example being a mark on the same block ashouses the sensor. Additionally, as shown in FIG. 3C, substrate table WTcan be moved to bring image sensor IAS1, IAS2 etc. into the position ofan aerial image 310, which is the projection through projection systemPS of a mark such as mark M1 on the mask MA. Electronic system 312detects properties of this aerial image 310, as it is received bysensors IAS1, IAS2, during translational movements of the substratetable WT, to locate in all degrees of freedom the exact location wherethe mask alignment mark M1 projects with optimum alignment (X-Y) andoptimum focus (Z) onto the sensor IAS1.

By way of the image sensors IAS1 and IAS2, when their position in thesubstrate table is well-known, the relative position of the aerial imageof the pattern on the mask MA with respect to the substrate table WT canbe determined. The substrate table WT may be provided with a substrate Wcomprising substrate marks, e.g. substrate marks P1, P2, P3, P4 asdepicted in FIG. 2. Alignment sensor 302, in co-operation with positionsensor IF, can obtain relative positions of the substrate marks P1, P2,P3, P4. The knowledge of the relative positions of the substrate marksP1, P2, P3, P4 can be obtained by the alignment sensor steps illustratedin FIGS. 3A and 3B. The relative position of the image of the objectmark on the mask MA with respect to the wafer table WT can be deducedfrom information obtained in a preliminary step with the image sensorsIAS1, IAS2, (FIG. 3C). These data allow the substrate W to be positionedat any desired position relative to the projected image of the mask MAwith great accuracy.

It must be understood that instead of two image sensors IAS1 and IAS2,more or fewer image sensors may be present, e.g. one or three. The formof these sensors and associated electronics is known to the skilledperson and will not be described in detail. Alternative forms ofalignment mechanism are possible, and useful within the scope of thepresent invention. Image alignment sensors IAS1, IAS2, may be mounted ona support separate from the table WT which carries the substrate,provided that their relative position can be determined.

FIG. 4 illustrates the principles of operation of the image alignmentsensors IAS1 and IAS2, in the known apparatus described above. In theknown apparatus, mask alignment mark M1 comprises sets of bright linesin the illumination pattern, some of them running in the X direction andthereby yielding Y position information, others running in the Ydirection and yielding X position information. In the examples thatfollow, only the detail of measurement in one of these directions willbe described, it being understood that the other direction can bemeasured likewise. Sensors for both X and Y directions can be combinedin one unit, and the marks can, in principle, be combined. Sets of marksat 45 degrees could be envisaged to derive mixed X and Y signals.

For simplicity, however, it is expected that X and Y directions will beprocessed side-by-side, either sequentially or in parallel. It shouldfurther be understood that a practical embodiment will involve processsteps for (i) bringing the sensor to the general location of theprojected mark, (ii) performing one or more coarse alignmentmeasurements, and (iii) finally obtaining the fine measurement.Different steps and processes, and/or different portions of the sensorand/or marks can be used in these different stages. For the sake ofillustration, FIG. 4 illustrates in fact a coarse alignment step (ii) inthe known apparatus, while the embodiments of the invention describedwith reference to FIG. 5 onwards concern especially the finemeasurements step, where the problem of accuracy is actually most acute.

AIM in FIG. 4 indicates generally the “aerial image” of three brightlines focused at a point in space with a depth of focus DOF on eitherside of a nominal Z position (the Z direction being the verticaldirection in the diagram). The horizontal axis in the diagram can beregarded as X or Y, as mentioned. The notation X/Y will be usedhereafter. The bright lines are represented in cross section by threeellipses, and have a known spacing. The image alignment sensor 400comprises a photodetector 402, housed in a body 404 (block or cavity)into which radiation from the patterned radiation beam can enter throughapertures 406. In this specific example the apertures 406 are three innumber, and they are spaced to correspond with the spacing of the threebright lines in the aerial image of the mask alignment mark. Theapertures may be defined, for example, by deposition of opaque material,for example a chrome metal layer 408. In the known sensor, a singlephoto diode 402 can receive and effectively integrate the radiationreceived through all three apertures, and along a line length extendingin the Y or X direction into the diagram.

The vertical (Z) position of the apertures and the Z position of theaerial image AIM are spaced apart by a distance EZ, which represents the“error” in a Z direction. Similarly, a plane 412 through the centre ofthe central bright line is spaced from a plane 414 through the centre ofthe central one of the apertures 406 by an X/Y direction error, EX/EY.As will be appreciated, the light entering body 404, and hence the lightintensity output by photodetector 402, will be at a maximum when thethree bright lines of the aerial image are maximally aligned with andfocused on the apertures 406 in the sensor 400, that is, when EZ andEX/EY are both zero. At the right hand side of the diagram, there isindicated schematically a scan path 420, extending horizontally in theX/Y direction, and extending by multiple passes also in the Z direction.The passes are labeled Z0, Z1, Z2 etc., and are made by moving thesubstrate table WT, on which the sensor 400 is fixedly mounted or towhich it is fixedly coupled.

The graph at the lower part of FIG. 4 is a plot of intensity versustime, as measured by photodetector 402. The periods of timecorresponding to scans Z0-Z4 are marked along the bottom axis. Withineach of these passes, three distinct peaks of intensity can be seen, twoweaker peaks on either side of a central, strongest peak. In each pass,weaker peaks arise when one or (in this case) two of the bright linesare aligned with two of the apertures 406 in the sensor 400. Such asituation corresponds approximately to the off-set situation illustratedat top left in FIG. 4. The strongest, central peak in each passcorresponds to the EX/EY=0 position, in which three bright lines arealigned with three apertures, allowing a maximum amount of radiation toimpinge on photodetector 402. Among different passes, it can be seenthat the intensity peaks corresponding to pass Z2 are the highest.Accordingly, by simple signal processing the position indicated by thespot on the graph axis and the depth value Z2 can be identified as theposition with optimum alignment in both the X/Y and Z (focus)directions. By plotting a curve of the highest peak across different Zvalues, a refined Z position between the scans can in principle bededuced.

By correlating the positions of this highest central peak with movementsignals received from the positioning subsystem which drives thesubstrate table WT, a datum can be established, by which, in principle,any position of the substrate table and substrate W can be achieved withrespect to the aerial image of the patterning device.

Naturally, this datum can only be used in conjunction with many othermeasurements to achieve the required accuracy of patterning on asubstrate. On the other hand, any inaccuracies of the datum positionswill undermine the accuracy of the entire process. In a practicalsystem, after the process illustrated in the graphs of FIG. 4, afurther, fine scanning process will be undertaken to identify exactlythe positions in X/Y and Z, to the accuracy required for the exposure ofdevice patterns of the substrate. As mentioned in the introduction, themotors and servo controllers which comprise the positioning subsystem PWare not optimized for this type of scanning, but rather are optimizedfor the exposure process which follows. Accordingly, errors are likelyto arise in the fine scanning process, which cannot be eliminated andmay limit the accuracy of the datum for subsequent measurements. Theimage alignment scanning process takes time, and this conflicts withthroughput requirements.

A number of proposals are made herein to address these sources of errorand delay, One type of solution which may be considered is to achievethe scanning of the image alignment sensors by actuators and/or servosystems independent of the substrate table WT itself. Accordingly,rather than being fixedly mounted to the substrate table, one mayenvisage sensor 400 and the like being mounted on a sub-table, driven inX/Y and/or in Z directions by, for example, voice coil or piezoelectricmotors. Photodetector 402 might remain static, while the opaque layer408 with apertures 406 only is moved from side to side and/orvertically. Combinations of such approaches may also be envisaged.

Some drawbacks with such proposals include the additional weight of themotors, position encoders and servo electronics necessary, as well astheir inherent complexity and cost. Accordingly, in the embodiments ofthe invention which are described below, an alternative approach istaken in which the image alignment sensor includes photodetectors andapertures which are entirely static in respect to the substrate tableWT, but are sub-divided in X/Y and/or in Z directions, to eliminateentirely the need for scanning movement, at least at the finemeasurement state. The sensors may be described as ‘self-scanning’,though in fact scanning is not a part of their operation, at least inthe mechanical sense.

Static Image Alignment Sensor

FIG. 5 illustrates a patterning device (reticle) MA having, for the sakeof example, mask alignment marks M1-M4 placed in its four corners,outside the device pattern areas 502. An enlarged view of mark M2, stillhighly schematic, is labeled 500. This mark includes a number of brightline features extending, some in the X direction and some in the Ydirection. These are labeled MIY and MIX respectively, to reflect thatlines MIX, aligned with the Y axis, provide X position information.Although only a single line is illustrated, various forms of markincluding lines and/or gratings may be envisaged, as will be furtherexplained below.

FIG. 6 illustrates a modified apparatus, which is presented as only asimple modification of the apparatus illustrated in FIGS. 1-3C. Themodification comprises a modified mask alignment mark 500 (of the typeshown in FIG. 5) and a modified image alignment sensor 504, in place ofsensor IAS1/IAS2/400, described above. Sensor 504 is still mountedstatically on the substrate table WT, and with a position accuratelyknown and/or measurable with respect to marks P1 etc on the substrate Witself. Modified signal processing circuitry 506 is indicated, togetherwith its connection to a further digital signal processor or computer600, having associated storage 602 and other connections. The divisionof tasks between the local signal processing circuitry 506 and computer600 is a matter of design choice with regard to performance and costrequirements, as well as issues of weight, mass and heat load on themoving table WT.

First Embodiment

FIG. 7 illustrates the principles of operation of this novel apparatusin a first embodiment. With regard to an X/Y direction, a sensor 700 isprovided, which has not a single photodetector, but an array ofphotodetector elements 702. These may be integrated on a substrate 704in the same manner as known camera sensors of line or square array type.Issues such as color sensitivity need not be a priority, in a typicalembodiment, while issues such as heat generation (power dissipation) maybe more significant in this application than in a consumer cameradevice. The photodiodes are sensitive to the wavelength of radiationused in the illumination and projection system. Readout circuitry 706may also be integrated on substrate 704, for sending individualintensity readings from the array of detector elements 702 to signalprocessor 708. Processor 708 incorporates schematically both theon-table processing circuitry 506 and the computer 600, as desired. Inthis first embodiment, processor 708 outputs a reading of the X/Y errorEX/EY, as a function of the current Z position. In relation to laterembodiments, it will be described how Z resolution can be incorporatedin the static sensor, and those techniques may be applied in thisembodiment also. Alternatively, the scanning movement in the Z directionmay be performed, in the same manner as the prior art, while the sensorremains static only in the X/Y direction.

The photodiodes may be directly exposed to the environment, or may becovered by layers for optical purposes and/or physical protection.Aperture grids may be provided to define the extent of thephotosensitive area more precisely, and/or to reduce crosstalk betweenelements.

In this first example, it is assumed that the mask alignment mark 710features a regular grating creating a substantially sinusoidal fringepattern in the plane of focus of a projection system PS. An intensitydistribution for the aerial image AIM is indicated schematically at 712.While the photodetector elements 702 do not move, because theireffective apertures are individually spaced in the X/Y direction, eachinherently responds to a different portion of the intensity profile 712.In the graph at bottom right in the diagram, the vertical axisrepresents the intensity I(i) measured at each photodetector with indexi. From the sample values plotted as small squares in the graph, andknowing that the intensity profile is sinusoidal, a curve 714 can beaccurately fitted to the samples measured, and the exact position of thepattern relative to the photo diode array of sensor 700 can becalculated and output by processor 708. It is understood that, becausethe fringe pattern is periodic, alias positions could be reported. Thiscan be addressed by sufficient accuracy in the prior, coarse positioningof the sensor 700. Alternative measures might include end-pointdetection, if the detector array is larger than the projected grating;alternatively, a finite grating has an envelope function convoluted withthe sinusoid intensity pattern, which can be used for coarse positionmeasurement

As mentioned already, the simple example illustrated in FIG. 7 isadapted only for measuring the X/Y position at a single Z position. Byscanning or other means such as will be described below, multiplesinusoidal curves can be plotted, for example the curve shown in dottedlines at 716. By fitting curves to the measured sample values atdifferent Z values, whether by scanning mechanically or otherwise, amaximum amplitude of the curve can be identified, which in turnindicates the EZ=0 position. It goes without saying, if there is anoffset between the effective aperture plane of the sensor 700 (or thesensor in any other embodiment) and other features on the substratetable WT and/or substrate W, they can be measured and added as an offsetto the datum for accurately positioning all parts in a knownrelationship.

It should be noted that the sensor 700 of FIG. 7 will only yield higherresolution than known sensors if the spacing of the individual pixeldetectors 702 is much smaller than the spacing of current generationgratings and image alignment sensors. Accordingly, improvements in imagesensor technology may be required before the FIG. 7 embodiment becomesattractive over the other embodiments to be described. Further issues tobe considered, beside those of heating and power dissipation, are theinevitable low signal to noise ratio (SNR) of small photo diodes,compared with the larger detector 402, used in the known system. Theseissues can be addressed by repeating measurements and integrating theresults, albeit that this may affect measurement time and throughput.

Second Embodiment

FIG. 8 illustrates the form and functional principles of a secondembodiment of novel sensor, based on a mark which creates an aerialimage AIM in the form of a discrete bright line 802, rather than aperiodic grating. The bright line is illustrated in this example runningparallel to the Y direction, for obtaining X position information. As inFIG. 7, a novel sensor 800 comprises a static block (typically asemiconductor substrate) on which is placed or formed a line array ofindividual photodetector elements 804. Readout circuitry 806 andprocessing circuitry 808 are provided, to obtain an EX/EY measurementfor a current Z value. Rather than being arrayed transversely to the Xdirection, it will be seen that the array of detector elements 804 isaligned at a shallow angle α with respect to the bright line (Y) axis.Thus the line of elements 804 is effectively distributed with a veryclose spacing over a certain range in X direction, while being spread ina relatively larger range along the Y direction, which contains noposition information in the case of the bright line 802. In this way avery fine pitch of sampling in the X direction can be obtained, withlarger, more practical and less noisy detector elements 804.

The upper graph in FIG. 8( a) illustrates by dark squares the samplevalues for intensity that may be received from the array. In thisexample, rather than fitting a periodic sine curve to the measuredsamples, a single peak is identified and fitted, as illustrated at 812.Processor 808 identifies a peak of this curve, potentially withsub-pixel accuracy, to output the value EX on the current value of Z. Asshown in the lower graph, a plurality of curves can be obtained fordifferent Z values, either by scanning or by other means, and a maximumcurve 814 selected to output both EX and EZ values for use as datumlevels in subsequent measurements and the ultimate exposure process. EYis obtained similarly from another sensor (not shown) and another markwith bright lines at right angles to the line 802. Note that, once thebright line is positioned over sensor 800 within the range of theelements 804, no scanning movement is required, no vibrations areinduced or positional servo errors, and therefore the accuracy of theimage alignment measurement results is limited only by the properties ofthe sensor & processor, and the stability of its fixing on substratetable WT.

It should be understood that a practical embodiment may use a maskalignment mark providing several bright lines 802. Still, the aim inthis embodiment is that these lines can be separated from each othervery clearly, rather than interfering in the manner of a grating.Several sensors 800 may be provided with separate substrates, or spacedlines of detector elements 804 may be provided across a singlesubstrate. The provision of multiple lines may improve capture range(coarse measurement speed is enhanced) and/or statistical combinationsof the results from plural arrays of elements 804 may be used to improvenoise rejection and accuracy in the final result. These details ofimplementation will be readily understood and applied by the skilledperson, and will not be specifically described and illustrated inrelation to this or further embodiments herein. As mentioned already,the same arrangement is repeated for measurements in the Y direction(EY).

FIGS. 8( b) and (c) show schematic cross sectional details of sensor 800in the longitudinal (Y) direction and a transverse (X) direction,respectively. It should be noted that FIG. 8 (b) presents only a smallnumber of elements 804 and apertures compared with the whole arrayillustrated in plan view. Moreover, while the X axis is indicated, thearray is of course aligned with an offset axis according to angle α, andit is this offset angle which makes the aerial image 802 impinge on someapertures and detectors but not others. Therefore, in this diagram andsimilar diagrams to follow, the labeling “X” and “Y” should beinterpreted loosely to refer to the direction X or Y plus or minus a,unless otherwise stated.

Going first to FIG. 8( b) a representation of the aerial image AIM ofthe bright line 802 is shown in broken elliptical lines. Individualdetector elements 804 are positioned at the base of a substrate body820, each beneath a respective aperture 822 formed in a chrome orsimilar opaque layer 824. Crosstalk between radiation entering oneaperture 822 and radiation received by the element 804 under aneighboring aperture should be minimized. This may be done by presentingthe elements 804 effectively as the aperture themselves, by positioningthem much closer than illustrated behind their apertures, and/or byproviding physical barriers of opaque and/or absorbent material betweenthe elements. These measures are not shown but all are within thecapability of the skilled person, and can be chosen according to theirconvenience, cost and relative performance.

FIG. 8( c) shows how each detector element 804 and aperture 822 may bebroadened in one direction (X/Y) relative to the other (Y/X), if thiswill maximize sensitivity to incident radiation without degrading theaccuracy of the measurement. For example, if aperture 822 is muchnarrower in the X direction than the brightest peak of the bright lineprofile 802, then accuracy will be lost through poor utilization of theincident radiation. On the other hand, if aperture 822 is wider than thepeak of intensity in the radiation profile of the bright line 802, thenspatial resolution will be lost, for no gain in utilization, SNR etc.

FIG. 9 shows, in a similar format to FIG. 8, a modified version ofsensor 800, in which the sensor block or substrate 830 is alignedexactly with the Y axis (potentially for ease of manufacture), while thearray of elements and/or apertures 804/822 is distributed obliquelyacross body 830 at the angle α. The signal processing and the signalsobtained as illustrated by the graphs in FIG. 9( a) are identical tothose in the FIG. 8 embodiment and will not be described in more detail.Some differences can be seen in that, referring to FIG. 9( b), we seeagain that the photodetector elements 804 while subdivided and small inthe longitudinal direction (Y), are broad for maximum light utilizationin the X direction. In the example shown in FIG. 9( c), body 830 carriesan array of detector elements which are all uniformly placed and are ofuniform width down the length of the sensor. Only the apertures 822 inthe opaque layer 824 are printed/etched so as to step progressively todifferent X positions. A roughly central one of the apertures 822 isillustrated in this cross-section, casting a cone of radiation 832 on acentral portion of a central one of the elements 804. An element nearerthe beginning of the array, for example, will receive a cone ofradiation 834 from an aperture (not shown in this cross-section) at anoffset position, and similarly to the other side as one progresses tothe other end of the array. It will be apparent that care should betaken to ensure that signals from apertures at the extremes of the arrayare not depressed by loss of radiation falling beyond the end of element804. Alternatively, a correction profile may be applied to the signalsread out and processed by processor 800, to compensate for any suchfalling off.

FIG. 10 illustrates a further modification similar to that of FIG. 9.Here, the same form of block 830 is used and the same processing. Theonly difference is that a single aperture stripe 822 is provided,running the length of the array of detector elements 804. At each pointalong the array, the aperture walls will have oblique portions as shownschematically at 834. Again, proximity between the opaque layer 824which contains the apertures and the detector elements 804 may be muchcloser in practice than the scale shown, so that crosstalk betweenelements is minimized. The term ‘vertical’ in this context is not usedin a strict sense, but is used to refer to the direction perpendicularto the substrate plane, which is usually the focus direction in alithography system.

Static Measurement of Z Error

While the examples described so far have addressed the measurement ofposition with respect to the X and/or Y directions, there will now bedescribed some modifications which allow the single, static sensor to beused for measurement, without mechanical scanning, of vertical (Z)alignment of a projected aerial image AIM. While these modifications aredescribed in the context of the second embodiment, they may be adaptedand applied equally to the first embodiment and the third embodiment(described below).

FIG. 11 illustrates the principle of this static measurement of Z error,based on a simple modification of the embodiment of FIG. 8. Sensor 900comprises a body 920 carrying an array of photo detecting elements 904and readout circuitry 906. The body 920 is mounted at an offset angle α,relative (in the case of X measurement) to the Y axis which is parallelto the bright line 902 projected by the mask alignment mark.

A modified signal processor 908 receives the individual pixel data fromreadout circuitry 906, and processes the data to generate not only an EXvalue, but also an EZ value. For this purpose, as shown in the insetdetail, the pixels (photodetector elements 904) are assigned todifferent depth values Z1, Z2, Z3 in a fixed sequence (or in a knownrandomized sequence). The manner of achieving a different depth valueper pixel is something which can be done in several ways, as describedfurther below. The number of different depth values provided may begreater or less than three. For the time being, it should just beunderstood that, rather than fitting a single intensity curve to thesample values received from photo detecting elements 904, processor 908uses the association between each pixel and the depth value Z1, Z2, Z3to treat these as three separate data sets, symbolized by the triangle,the open rectangle and the black rectangle, as plotted in the graph atthe foot of FIG. 11.

Fitting curves of the expected shape to each of the three sample setsreveals three intensity/sample position profiles, labeled 912-1, 912-2and 912-3. As indicated, these curves correspond respectively to the Z1,Z2, Z3 depth values. As curve 912-3 is the highest in peak intensity, avalue EZ corresponding to Z3 can be output, as well as a value Xcorresponding to the X position of the peak of the fitted curve.Provided that the curve shape is known and fitted to a number ofsamples, sub-pixel resolution can be obtained. It will be appreciatedthat, in providing for resolution between only three values, Z1, Z2 andZ3 in the Z direction, resolution in the X direction has been divided bythree, at least in each data set. Having said that, on the assumptionthat the intensity distribution in the aerial image 902 is not tiltedbut is symmetrical about a plane perpendicular to the X axis, Xinformation from the weaker curves 912-1 and 912-2 can be combined withthat from the strongest curve 912-3 to improve accuracy of the Xposition. Moreover, although not plotted against the Z direction, curvescan be fitted also between the heights of the peaks and resolution finerthan the steps Z1, Z2, Z3 can be obtained in the Z dimension also.

Noting that the highest peak corresponds to Z3, which is an extremerather than a middle one of the Z values available, one shouldappreciate that better certainty in the Z value will be obtained ifsamples are obtained from either side of the peak value. In a practicalembodiment, there may be more than three levels of Z. In any event, themeasurement shown can be used as a coarse result, and the substratetable WT and sensor 900 can be moved to a slightly different level, andfixed there while a measurement in which peak 912-2 (for depth Z2) isthe highest of the three is performed. Again, because the method doesnot involve mechanical scanning during measurement (at least at the fineresult stage), accuracy is improved. Speed may be improved also.

FIG. 12 illustrates one technique for obtaining parallel measurementsfor different Z values, without moving sensor 900. As in the earlierdrawings, FIG. 12( a) shows a longitudinal cross section of a part ofthe sensor, while FIG. 12( b) shows a transverse cross section. Asbefore, some form of body 920 is provided, which houses individualphotodetector elements 904, beneath apertures 922 in an opaque layer924. The modification which places each pixel effectively at a differentZ value is to vary the optical path length between neighboring aperturesand a datum line 940, by the insertion of differing thicknesses ofmaterial having another refractive index than the environment. Where therefractive index is n, the optical path length in a real distance dz isn.dz. Naturally, the optical path length and refractive index have to becalculated with respect to the actual wavelength of radiation used, andthe refractive index of any immersion medium (water) that may be presentin place of air or vacuum.

The different step heights are shown in FIG. 12 (a), side by side, whileFIG. 12 (b) represents a small cross section through a step 942-2 at theheight corresponding to level Z2. Other step heights are shown in brokenlines as 942-1 and 942-3.

FIG. 13 shows a modified sensor 900 with several lines of photodetectors904. Each line in this case has its own Z value, for example four linesat depths Z1-Z4. The processing to obtain results EX and EZ is simply aversion of the processing described with respect to FIG. 11, adapted forthe fact that the apertures/detectors for each Z value are displaced bya known amount to one another, particularly in the X direction.Therefore a specific offset in the X direction may be applied to theresult from each different line of pixels. The EX value can be used fromthe line having the strongest intensity, or one can combine them all,with their individual offsets, to arrive at an agreed accurate EX value.

FIGS. 13( b) and (c) show the implementation of the four Z values acrossthe sensor 900, according to which the “staircase” of refractivematerial thickness is not repeated cyclically along the length of thesensor, but rather is constant along the length, and progresses in foursteps from line to line of pixels, along the line C-C′. FIG. 13( c)shows also that the detector elements 904 are now sub-divided in thetransverse direction, as well as in the longitudinal direction. In bothembodiments (FIG. 12, FIG. 13), issues concerning the management ofcrosstalk, resolution, light capture etc apply, as they did in examplesof FIGS. 8-10.

FIG. 14 shows a couple of alternative approaches to the creation of the“staircase” in the Z direction. In both FIGS. 14( a) and (b) the opaquelayer creating the apertures 922 is applied on top of the steprefractive material, instead of below it. FIG. 14( b) also shows anexample in which detector elements 904 are on top of a substrate body950, rather than spaced through a body material at a distance from theapertures and refractive material.

The refractive material may comprise etched glass as one example, or itmay comprise silicon dioxide, such as may be integrated on top of aphotodetector, by normal semiconductor processing. Many variations andpermutations of the measures described can be applied, all with the aimof imparting different optical path lengths from one pixel to another.Instead of a staircase structure, particularly in the FIG. 13 example itmay be more practical to create a sloping wedge profile. If the angle ofthe wedge is such as to effectively displace the beams by a prismeffect, this can be compensated by an offset in the signal processing toobtain true X values. Rather than a wedge of refractive material, itwould be possible alternatively to mount the detector array at apre-arranged Z tilt (known as RX, for rotation about the X axis, or RYfor rotation about the Y axis). This will inherently place the aperturesat different heights relative to the plane of the substrate table WTwithout special refractive material. Instead of varying the thickness ofthe material, a constant thickness of materials having differentrefractive indices could be applied. Different materials may bedeposited separately, or the refractive index of a single layer may bemodified by doping different portions. These various approaches can evenbe combined to achieve the desired variation in optical path length.

FIG. 15 shows a further way to impart different effective Z values toneighboring pixels in the array of elements 904 in a detector body 920which is neither tilted nor provided with steps or wedges of refractivematerial. In this example, the mask alignment mark M1, M2 etc. ismodified as at 960, to include the steps or wedge of varying Z height.This can be achieved by applying or etching away a certain depth ofglass or other refractive material, or of variably doped materials andthe like, as described above. The result of this is that adjacent brightlines in the aerial image AIM are focused at slightly different heights,labeled 902-1 (Z1), 902-2 (Z2), 902-3 (Z3) and 902-4 (Z4) in thediagram. Accordingly, when the apertures 922 of different lines ofdetector elements 904 are presented to the aerial image at a levelsomewhere among the Z levels of the aerial images of the bright lines,information on the relative magnitudes of four Z error values EZ1-EZ4can be obtained, and a curve fitted to identify the Z error EZ from asingle static measurement step. Naturally the need for a speciallyprocessed reticle makes this embodiment less convenient for opticallithography applications.

Third Embodiment

FIG. 16 shows a third type of embodiment in which a sensor 1000 includesrelatively sparsely placed detector elements 1004. Readout circuitry1006 and processor 1008 are provided, as in previous embodiments. Eachdetector element may comprise a line to collect a good quantity oflight. Each element may be behind an aperture, or the nature of thedetector itself may define an effective aperture of the appropriate sizeand shape. Viewed in only the X/Y direction, it can be seen that theelements 1004 are relatively sparsely placed. In the orthogonaldirection (into the page) they may be a continuous line, or a line ofdiscrete detector elements 1004. In the simple example illustrated, Zinformation is not obtained directly, but the principles of providingfor different Z path lengths to different detectors can be applied inthis embodiment equally as they are in the embodiments of FIGS. 11-15.The elements 1004 may be sub-divided in the orthogonal direction forthis purpose.

The mask alignment mark 1060 in this case is arranged to deliver,through projection system PS, a series of bright lines, represented byspikes in the intensity profile 1002 representing the intensity of theaerial image AIM. The spatial period of these bright lines is close tobut slightly different from the spacing of the detector elements 1004,so that only one element at a time can receive the full intensity of abright line. This vernier-like arrangement yields sample values such asare illustrated in the graph at the foot of FIG. 16, from which a curve1012 can be inferred, and a value for EX/EY can be derived from thecentre line 1016 of that curve.

Variants & Applications

As mentioned already, the various features of the embodiments describedabove can be applied in X and Y directions, independently or in acombined form. FIG. 17 illustrates a sensor block 1100 which containstwo fine arrays of photo-detecting elements forming a sensor 1102 forX-direction information and a sensor 1104 for Y-direction positioninformation. Each of these sensors is illustrated with the form similarto the example of FIG. 13, but could equally be a sensor of the formillustrated in any of the embodiments from FIG. 8 to FIG. 16. Readoutcircuitry 1106, 1108 provides signals to processors 1110 and 1112. Eachsensor 1102, 1104 also includes Z-direction resolving capability, andthe processors 1110 and 1112 together provide Z-position information(EZ). Readout circuitry 1106, 1108 and/or processors 1110 and 1112 neednot be provided separately for the X- and Y-directions, but could becombined as desired.

Also on sensor block 1100 is a coarse alignment sensor 1120, with itsown readout circuitry 1122 and processor 1124. Processor 1124 outputscoarse error signals EC, with X, Y and Z components, which are used bythe substrate table positioning system PW to bring the fine sensors1102, 1104 into alignment with the mask alignment features, in this casebright lines 1126, 1128. For this purpose, the projected mask alignmentpattern also includes coarse alignment features, such as the crossfeature 1130. As can be seen, once the cross feature 1130 is alignedcentrally on the coarse alignment sensor 1120, the bright lines 1126 and1128 will be centered on their respective X and Y sensors 1102, 1104,and further movement of the substrate table is not necessary to obtainthe fine readings EX, EY, EZ. Additionally or alternatively, correctionsto the coarse position can be triggered by signals from the fineposition processors 1122, 1124, as indicated by the dashed arrow.

Trade-offs exist in the accuracy achievable between all these variants,in terms of the complexity of processing, and the compactness and costof the sensors. While bright lines are described as alignment features,dark lines on a bright field could in principle also be used.

The fine measurement photodetectors may be integrated also with coarsepositioning photodetectors, arranged to respond to the same projectedfeatures over a wider range of EX, EY and/or EZ or to additionalfeatures. Photodetectors for coarse positioning may be entirely separatefrom those used for fine positioning, or may be shared. Where a line ofdetectors is arranged at a shallow angle α for the fine measurement, aline of broader detectors can be arranged at a greater angle for coarsecapture and measurement, still without scanning movements.

FIG. 18 illustrates a further alternative sensor block 1150 in which asensor 1152 comprises a unified, 2-dimensional array of photo-detectorelements. For this example, a two-dimensional mask alignment pattern1154 is generated, for example crossed lines rather than simple parallellines, and the single detector array can resolve both X and Yinformation by appropriate processing. Given a sufficiently fine pixelpitch, the array could be squarely aligned with the X, Y grid, similarto the example of FIG. 7. For reasons given above, however, setting thearray at an angle relaxes the sensor pitch requirement and simplifiessignal processing.

Many other forms of mark and coarse positioning systems can beenvisaged, and the example illustrated is in no way intended to belimiting. The example is also not intended to be to scale. For practicalreasons, sensor block 1100 is likely to be much larger in extent thanthe sensors 1102, 1104 within it. This is particularly the case wherecoarse alignment sensors and other sensors are mounted in the samemodule. The extent of the entire alignment pattern may be on the orderof a millimeter or a few millimeters. Feature sizes and spacings in thealignment pattern itself may be of the order of a micron or a fewmicrons. The spatial resolution achieved by curve fitting, averaging andother techniques can be finer than the feature size, and finer than thewavelength of radiation employed. The feature size may be smaller still,in the case of an EUV or imprint apparatus having naturally tighterpositioning requirements.

The skilled person will appreciate that aperture, as used throughoutthis section, can mean both a single line (that is a single rectangulartransmission function) as well as a more complex line pattern (that is amore complex localized transmission function, which on average resemblesa rectangular transmission function).

In an embodiment, sensors such as sensor 700, 800, 900, 1000, 1100 and1150 are provided at two, three or four positions on the substrate tableWT, so as to read two or more of the mask alignment marks M1-M4simultaneously. While X and Y may be measured simultaneously, theinvention can give a speed and accuracy advantage, even in embodimentswhere EX, EY and EZ are measured, sequentially or two at a time.

While certain embodiments measure different Z values simultaneouslyusing different pixels interleaved in a repetitive pattern or inparallel lines, more complex interleaving patterns are possible, whetherin one-dimensional or two-dimensional arrays. Another option for varyingthe optical path length between samples would be a moving refractiveelement such as a segmented wheel or other moving part. This would needto be designed so as not to disturb the X and Y measurements, however,if they are to made strictly simultaneously. The Z variations can bemeasured at a separate time, however, and the benefit of not using thesubstrate table or mask table motors for scanning in Z would beretained.

While direct projection between alignment mark and the sensor isillustrated and described in the embodiments that follow, modificationsare envisaged in which for some reason the projection of the alignmentmark is reflected at one or other of the substrate support and thepatterning device or its support, and the sensor and alignment mark areboth at the same side of the projecting optical system.

These and many other variations can be envisaged by the skilled reader,based on the present disclosure.

It should be understood that the processing unit 600 in the previousembodiments may be a computer assembly as shown in FIG. 19. The computerassembly may be a dedicated computer in the form of a control unit inembodiments of the assembly according to the invention or,alternatively, be a central computer controlling the lithographicprojection apparatus. The computer assembly may be arranged for loadinga computer program product comprising computer executable code. This mayenable the computer assembly, when the computer program product isdownloaded, to control aforementioned uses of a lithographic apparatuswith embodiments of the image alignment sensors 700, 800 etc.

Memory 1229 connected to processor 1227 may comprise a number of memorycomponents like a hard disk 1261, Read Only Memory (ROM) 1262,Electrically Erasable Programmable Read Only Memory (EEPROM) 1263 enRandom Access Memory (RAM) 1264. Not all aforementioned memorycomponents need to be present. Furthermore, it is not essential thataforementioned memory components are physically in close proximity tothe processor 1227 or to each other. They may be located at a distanceaway

The processor 1227 may also be connected to some kind of user interface,for instance a keyboard 1265 or a mouse 1266. A touch screen, trackball, speech converter or other interfaces that are known to personsskilled in the art may also be used.

The processor 1227 may be connected to a reading unit 1267, which isarranged to read data, e.g. in the form of computer executable code,from and under some circumstances store data on a data carrier, like afloppy disc 1268 or a CDROM 1269. Also DVD's or other data carriersknown to persons skilled in the art may be used.

The processor 1227 may also be connected to a printer 1270 to print outoutput data on paper as well as to a display 1271, for instance amonitor or LCD (Liquid Crystal Display), of any other type of displayknown to a person skilled in the art.

The processor 1227 may be connected to a communications network 1272,for instance a public switched telephone network (PSTN), a local areanetwork (LAN), a wide area network (WAN) etc. by way oftransmitters/receivers 1273 responsible for input/output (I/O). Theprocessor 1227 may be arranged to communicate with other communicationsystems via the communications network 1272. In an embodiment of theinvention external computers (not shown), for instance personalcomputers of operators, can log into the processor 1227 via thecommunications network 1272.

The processor 1227 may be implemented as an independent system or as anumber of processing units that operate in parallel, wherein eachprocessing unit is arranged to execute sub-tasks of a larger program.The processing units may also be divided in one or more main processingunits with several sub-processing units. Some processing units of theprocessor 1227 may even be located a distance away of the otherprocessing units and communicate via communications network 1272.

It is observed that, although all connections in FIG. 19 are shown asphysical connections, one or more of these connections can be madewireless. They are only intended to show that “connected” units arearranged to communicate with one another in some way, The computersystem can be any signal processing system with analogue and/or digitaland/or software technology arranged to perform the functions discussedhere.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured. For imprint lithography, thereis no projection system required to transfer the product pattern itselffrom the patterning device to the substrate. Nevertheless, opticalsystems can be employed to project an alignment pattern from thepatterning device to image alignment sensors of the type describedherein. The invention is not limited to optical lithography. The terms“radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention, or features within theinvention, may take the form of a computer program containing one ormore sequences of machine-readable instructions describing a method asdisclosed above, or a data storage medium (e.g. semiconductor memory,magnetic or optical disk) having such a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus arranged to transfer a pattern from apatterning device onto a substrate, the apparatus comprising: apatterning subsystem for receiving said patterning device and applyingsaid pattern to the substrate held at a patterning location; a substratesupport for holding the substrate while said pattern is applied; atleast one positioning subsystem for moving said substrate supportrelative to said patterning subsystem and said patterning device suchthat said pattern is applied at an accurately known location on thesubstrate; and a measuring subsystem for measuring the location of saidsubstrate relative to the patterning location, and for supplyingmeasurement results to said positioning subsystem, wherein saidmeasuring subsystem includes at least one sensor for receiving radiationprojected from an alignment mark, the sensor and the alignment markbeing associated one with the patterning device and the other with thesubstrate support, and a processor for receiving and processing signalsfrom the sensor to resolve spatial information in the projectedalignment mark to establish a reference position for measuringpositional relationships between said substrate support and saidpatterning location, wherein said sensor comprises an array ofphotodetector elements separated in at least one dimension and arrangedin a line, the line being oriented at a shallow non-zero angle to theorientation of a line in the projected alignment mark such that thesensor and the processor are operable to perform at least a final stepin establishing the reference position while holding the substratesupport and patterning device stationary with respect to one another andthe processor calculating from signals of said array a referenceposition in a dimension perpendicular to the line in the projectedalignment mark.
 2. An apparatus as claimed in claim 1 wherein saidsensor comprises at least one array of photodetector elements spacedapart in a first dimension, the processor calculating from signals ofsaid array the position in said first dimension of the projectedalignment mark which comprises features having a different spacing insaid first dimension.
 3. An apparatus as claimed in claim 1 wherein aplurality of sensors are provided for measuring reference positionsrespectively in different dimensions.
 4. An apparatus as claimed inclaim 1 wherein said shallow non-zero angle is an angle less than about30 degrees.
 5. An apparatus as claimed in claim 1 wherein said sensor isassociated with said substrate support, while the alignment mark isassociated with the patterning device.
 6. An apparatus as claimed inclaim 1 wherein said sensor is operable to measure reference positionsin three dimensions simultaneously while the substrate support is heldstationary.
 7. A lithographic apparatus as in claim 1, wherein each ofthe photodetector elements of the array of photodetector elements isconfigured and arranged to detect a respective spatially separatedportion of a spatial distribution of intensity of the received radiationfrom the alignment mark, and the sensor and the processor are operableto perform said final step in establishing the reference position on thebasis of the detected spatial distribution of intensity.
 8. A devicemanufacturing method comprising transferring a pattern from a patterningdevice onto a substrate, the method comprising: providing a patterningsubsystem for receiving said patterning device and applying said patternto a portion of said substrate held at a patterning location; holdingthe substrate on a substrate support; measuring the location of saidsubstrate relative to the patterning location; operating said patterningsubsystem while using results of said measuring step to position saidsubstrate support, said patterning subsystem and said patterning devicerelative to each other in a sequence of movements such that said patternis applied at a desired portion of the substrate; and processing saidsubstrate to create product features in accordance with the appliedpattern, wherein said measuring step includes a preliminary step of (i)receiving radiation projected from an alignment mark using a sensorwhich comprises an array of photodetecting elements separated in atleast one dimension and arranged in a line, the line being oriented at ashallow non-zero angle to the orientation of a line in the projectedalignment mark and (ii) processing signals from the sensor to resolvespatial information in the projected alignment mark to establish areference position for measuring positional relationships between saidsubstrate support and said patterning location in at least said onedimension, such that the sensor and the processor perform at least afinal step in establishing the said reference position while thesubstrate support and patterning device are held stationary with respectto one another and the processor calculating from signals of said arraya reference position in a dimension perpendicular to the line in theprojected alignment mark.
 9. A method as claimed in claim 8 wherein atleast two sensors are provided for measuring reference positionsrespectively in two orthogonal directions generally parallel to asubstrate plane, one or both of said sensors additionally providingmeasurement in a third dimension parallel to an optical axis of theprojection system.
 10. A method as claimed in claim 8 wherein for theperformance of said preliminary step said sensor is associated with saidsubstrate support, while the alignment mark is associated with thepatterning device.
 11. A method as claimed in claim 10 wherein, givencompatible alignment marks on the pattering device, said sensor(s) areoperable in the preliminary step to measure reference positions in threedimensions simultaneously while the substrate table and patterningdevice are held stationary relative to one another.
 12. A method asclaimed in claim 8, wherein each of the photodetector elements of thearray of photodetecting elements is configured and arranged to detect arespective spatially separated portion of a spatial distribution ofintensity of the received radiation from the alignment mark, and theprocessing signals from the sensor to resolve spatial informationcomprises using the spatial distribution of intensity of the receivedradiation.
 13. A method as claimed in claim 8, wherein said shallownon-zero angle is an angle less than about 30 degrees.
 14. Anon-transitory machine readable medium containing one or more sequencesof machine-readable instructions for controlling a lithographicapparatus, the instructions being adapted for controlling themeasurement and positioning steps of a method as claimed in claim 9, andfurther to cause one or more programmable processors of the apparatus toprocess signals from the sensor to resolve spatial information in theprojected alignment mark to establish a reference for measuringpositional relationships between said substrate support and saidpatterning location in at least one dimension, at least a final step inestablishing the said reference position being performed while thesubstrate support is held stationary.
 15. A non-transitory machinereadable medium as in claim 14, wherein each of the photodetectorelements of the array of photodetecting elements is configured andarranged to detect a respective spatially separated portion of a spatialdistribution of intensity of the received radiation from the alignmentmark, and the processing signals from the sensor to resolve spatialinformation comprises using the spatial distribution of intensity of thereceived radiation.
 16. A lithographic apparatus arranged to transfer apattern from a patterning device onto a substrate, the apparatuscomprising: a patterning subsystem for receiving said patterning deviceand applying said pattern to the substrate held at a patterninglocation; a substrate support for holding the substrate while saidpattern is applied; at least one positioning subsystem for moving saidsubstrate support relative to said patterning subsystem and saidpatterning device such that said pattern is applied at an accuratelyknown location on the substrate; and a measuring subsystem for measuringthe location of said substrate relative to the patterning location, andfor supplying measurement results to said positioning subsystem, whereinsaid measuring subsystem includes at least one sensor for receivingradiation projected from an alignment mark, the sensor and the alignmentmark being associated one with the patterning device and the other withthe substrate support, and a processor for receiving and processingsignals from the sensor to resolve spatial information in the projectedalignment mark to establish a reference for measuring positionalrelationships between said substrate support and said patterninglocation, wherein said sensor comprises an array of photodetectorelements separated in at least one dimension and arranged in a line, theline being oriented at a shallow non-zero angle to the orientation of aline in the projected alignment mark such that the sensor and theprocessor are operable to perform at least a final step in establishingthe reference position while holding the substrate support andpatterning device stationary with respect to one another wherein theprocessor is arranged to distinguish between the different photodetectorelements in accordance with respectively different optical path lengthsfrom the alignment mark, thereby to calculate a reference position in adimension (Z) parallel to an optical axis of a projection system of thelithographic apparatus.
 17. An apparatus as claimed in claim 16, whereinat least two groups of photodetector elements are provided for measuringreference positions respectively in two orthogonal directions generallyparallel to a substrate plane, one or both of said groups additionallyproviding measurement in a third dimension parallel to the optical axisof the projection system.
 18. An apparatus as claimed in claim 6,wherein optical path length differences between the photodetectorelements are imparted by varying one or both of the thickness andrefractive index of material placed in front of the elements.